A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may for example project a pattern from a patterning device (e.g. a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate.
The wavelength of radiation used by a lithographic apparatus to project a pattern onto a substrate determines the minimum size of features which can be formed on that substrate. A lithographic apparatus which uses EUV radiation, being electromagnetic radiation having a wavelength within the range 4-20 nm, may be used to form smaller features on a substrate than a conventional lithographic apparatus (which may for example use electromagnetic radiation with a wavelength of 193 nm).
EUV radiation may be produced using a radiation source arranged to generate an EUV producing plasma. An EUV producing plasma may be generated, for example, by exciting a fuel within the radiation source. In addition to generation of plasma, exciting the fuel may also result in the unwanted generation of particulate debris from the fuel. For example, where a liquid metal, such as tin, is used as a fuel, some of the liquid metal fuel will be converted into an EUV producing plasma, but debris particles of the liquid metal fuel may be emitted at high speeds from the plasma formation region. The debris may be incident on other components within the radiation source, affecting the ability of the radiation source to generate an EUV producing plasma or to provide a beam of EUV radiation from the plasma to other components of the lithographic apparatus. The debris may also travel beyond the radiation source and become incident on other components of the lithographic apparatus.
The presence of tin or other deposited debris on the collector of an EUV radiation source can cause particular difficulties, as usually the collector is configured for optimal operation in the absence of tin or other debris deposited on the surface of the collector. The presence of tin on the collector surface can, for example, affect the reflection coefficient of the collector at EUV wavelengths.
The collector of an EUV radiation source can, in operation, be exposed to high fluxes of tin. The tin can be cleaned away by EUV-induced cleaning during operation. However, spots of tin remain on the collector, which can reduce the reflectivity of the collector and eventually lead to a requirement to replace the collector.
The need periodically to replace the collector in an EUV radiation source can lead to a reduced availability of the EUV radiation source and associated lithographic system, increased costs, and increased technical and operational burden in removing, transporting and cleaning collectors.
It has been suggested that EUV-induced cleaning of collectors in situ in an EUV radiation source during operation may be caused, or at least assisted, by the presence of secondary electrons at or near the collector surface.